A typical semiconductor optical amplifier (SOA) 900, as shown in FIG. 9, is a wave-guide structure with a semiconductor gain medium 106 (either bulk or multi-quantum well), similar to a semiconductor laser. Semiconductor gain medium 106 is sandwiched between a substrate 104 and a semiconductor layer 902. These two layers, 104 and 902, have a lower index of refraction than gain medium 106 and tend to confine the optical mode within gain medium 106, as does passivation layer 114. Passivation layer 114, which is generally formed of an insulating dielectric material, serves to protect the waveguide and substrate surfaces and reduce surface leakage currents, as well as to act as a cladding layer. Contact layer 904 desirably provides reduced contact resistance with contact 906 and provides a ready supply of carriers to be pumped into gain medium 106 during operation giving rise to a population inversion. Stimulated radiative recombination of carriers in gain medium 106 leads to coherent amplification of optical signals passing through the SOA. The ends of the wave-guide are usually treated to avoid optical feedback. This treatment may include, for example applying an antireflection coating, or forming a tilted mirror, buried facet, etc. Therefore, an SOA operates as a traveling wave amplifier with its gain controlled by current injection from contact layer 904.
In order to achieve high gain and low power drive, the devices have a wave-guide mode, which defines an interaction region 908 of amplification layer 106. Interaction region 908 has a width matched to mesa width 120. The contact layer 904 and the electrical contact 906 also match this width so that current injection is ensured across the full mesa width. If the desired amplification outpaces the rate at which carriers may be pumped into interaction region 908, carrier depletion within the interaction region may result, reducing the gain of the SOA.
One important limitation of these amplifiers in high-speed optical communications systems is the axial carrier depletion induced by the leading edges of pulses, which results in undesired pulse distortion and self phase modulation. This carrier depletion may additionally vary along the length of the SOA, which may lead to further distortion of the output optical signal. To avoid this distortion, stringent output power limits have generally been imposed, limiting the applications of SOA's. Thus, a technique or design that reduces carrier depletion may be very desirable.
In U.S. Pat. No. 4,939,474, Eisenstein et al. disclose an SOA with shortened gain-recovery time. In the disclosed SOA, an amplification layer, which is either undoped or lightly n-doped, has an interaction region and a carrier-storage region adjacent to the interaction region of the amplifier. Passage of carriers from the storage region to the gain region is used to replenish the carrier population within the gain region, thereby permitting recovery of the amplifier gain. This method provides somewhat decreased carrier depletion in the interaction region due lateral diffusion of carriers within the amplification layer. The rate of gain-recovery is largely determined by the diffusion rate for holes in the amplification layer, as holes generally diffuse much more slowly than electrons.